1. Field of the Invention
The present invention relates to a water electrolysis system, which has a high-pressure hydrogen production unit for electrolyzing water to generate oxygen at an anode side and hydrogen at a cathode side, and a hydrogen supply pipe for supplying hydrogen generated in the production unit. The present invention also relates to a method for operating such a system.
2. Description of the Related Art
In fuel cells, hydrogen generally is used as a fuel gas for performing a power generation reaction. For example, a water electrolysis apparatus is used to produce hydrogen. The water electrolysis apparatus contains a solid polymer electrolyte membrane (an ion-exchange membrane) for decomposing water to generate hydrogen (and oxygen). Electrode catalyst layers are formed on either side of the solid polymer electrolyte membrane to thereby prepare a membrane-electrode assembly, and current collectors are placed on either side of the membrane-electrode assembly to produce a unit cell.
A plurality of such unit cells are stacked, a voltage is applied to respective ends of the cell stack in the stacking direction, and water is supplied to the anode-side current collector. Then, the water is decomposed to generate hydrogen ions (protons) at the anode side of the membrane-electrode assembly. The hydrogen ions permeate through the solid polymer electrolyte membrane to the cathode side, and become bonded with electrons to produce hydrogen. Meanwhile, at the anode side, oxygen generated simultaneously with the hydrogen is discharged together with residual water from the cell stack.
Hydrogen generated by the water electrolysis apparatus contains water. A hydrogen product for a fuel cell vehicle or the like is required to be in a desired dry state (to have a desired water concentration). For example, the product comprises hydrogen having a water amount of 5 ppm or less (hereinafter referred to as dry hydrogen).
For example, a known dehumidification mechanism for removing water contained in hydrogen is disclosed in Japanese Laid-Open Patent Publication No. 2004-149890. As shown in FIG. 23, the dehumidification mechanism contains a dehumidification unit 6. The dehumidification unit 6 has a main vessel body 2, a dehumidifying agent 1 for dehumidifying untreated gas contained in the main vessel body 2, and a hydrogen gas supply pipe 3a and a hydrogen gas discharge pipe 3b connected to lower and upper ends of the main vessel body 2. The dehumidification unit 6 further has a cooling trace 4 for circulating a cooling gas, which is wound helically at approximately regular intervals on the outer surface of the main vessel body 2, and a heating wire 5, which is arranged within the cooling trace 4 parallel and adjacent to the cooling trace 4.
In the dehumidification mechanism, hydrogen gas generated by electrolysis is transferred to the dehumidification unit 6 in a dehumidification step, and is introduced into the main vessel body 2 through the lower hydrogen gas supply pipe 3a. The hydrogen gas is dehumidified to a predetermined dew point by the dehumidifying agent 1, and then is discharged from the upper hydrogen gas discharge pipe 3b to the outside of the main vessel body 2, and is supplied to a hydrogen storage unit such as a hydrogen tank.
The recovery process of the dehumidification unit 6 contains the steps of heating the dehumidifying agent 1 to remove water, and cooling the heated dehumidifying agent 1 to an approximately normal temperature. More specifically, in the heating step, the entire main vessel body 2 is heated by the heating wire 5. In the cooling step, cooling gas is introduced into the cooling trace 4, whereby the dehumidifying agent 1 is cooled to regain a predetermined dehumidification capability.